Protein kinase C activation stabilizes LDL receptor mRNA via the JNK pathway in HepG2 cells.
LDL is the most abundant cholesterol transport vehicle in plasma and a major prognostic indicator of atherosclerosis. Hepatic LDL receptors limit circulating LDL levels, since cholesterol internalized by the liver can be excreted. As such, mechanisms regulating LDL receptor expression in liver cells are appealing targets for cholesterol-lowering therapeutic strategies. Activation of HepG2 cells with phorbol esters enhances LDL receptor mRNA levels through transcriptional and posttranscriptional mechanisms. Here, we show that 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced stabilization of receptor mRNA requires the activity of protein kinase C and is accompanied by activation of the major mitogen activated protein kinase pathways. Inhibitor studies demonstrated that receptor mRNA stabilization is independent of the extracellular signal-regulated kinase or p38(MAPK), but requires activation of the c-Jun N-terminal kinase (JNK). An essential role for JNK in stabilizing receptor mRNA was further confirmed through small interfering RNA (siRNA) experiments and by activating JNK through two protein kinase C-independent mechanisms. Finally, prolonged JNK activation increased steady-state levels of receptor mRNA and protein, and significantly enhanced cellular LDL-binding activity. These data suggest that JNK may play an important role in posttranscriptional control of LDL receptor expression, thus constituting a novel mechanism to enhance plasma LDL clearance by liver cells.
Vargas
Noelle B
NB
Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD 21201.
Brewer
Brandy Y
BY
Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD 21201.
Rogers
Terry B
TB
Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD 21201.
Wilson
Gerald M
GM
Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD 21201; Marlene and Stewart Greenebaum Cancer Center, University of Maryland School of Medicine, Baltimore, MD 21201. Electronic address: gwils001@umaryland.edu.
D002460:Cell Line; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D008099:Liver / Q000166:cytology / Q000378:metabolism; D048688:MAP Kinase Kinase 7 / Q000235:genetics / Q000378:metabolism; D020935:MAP Kinase Signaling System; D008954:Models, Biological; D009154:Mutation; D011493:Protein Kinase C / Q000378:metabolism*; D012323:RNA Processing, Post-Transcriptional; D020871:RNA Stability; D012333:RNA, Messenger / Q000235:genetics* / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000235:genetics; D011973:Receptors, LDL / Q000235:genetics*; D015398:Signal Transduction; D014161:Transduction, Genetic; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
D012333:RNA, Messenger; D034741:RNA, Small Interfering; D011973:Receptors, LDL; D011493:Protein Kinase C; D048049:Extracellular Signal-Regulated MAP Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D048051:p38 Mitogen-Activated Protein Kinases; D048688:MAP Kinase Kinase 7; C482569:MAP2K7 protein, human